Processes for Producing Acetic Acid

ABSTRACT

Processes for the producing acetic acid and, in particular, to improved processes for removing a cation, such as lithium, and iodides from a low energy carbonylation process to produce purified acetic acid. In one embodiment, the cation, e.g., lithium, may be removed using a cationic exchanger prior to removing iodides using a metal-exchanged ion exchange resin. The present invention is also suited for removing at least one cation selected from the group consisting of Groups IA and IIA of the periodic table, quaternary nitrogen cations, and phosphorous-containing cations.

CROSS-REFERENCES TO RELATED APPLICATIONS

This patent application is a continuation-in-part and claims the priority of U.S. patent application Ser. No. 14/694,913, entitled “Processes For Producing Acetic Acid,” filed Apr. 23, 2015, and claims priority from U.S. Provisional Patent Application Ser. No. 62/141,490, entitled “Processes For Producing Acetic Acid,” filed Apr. 1, 2015, the disclosure of which are incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to processes for producing acetic acid and, in particular, to improved processes for removing lithium cations, which may be derived from and/or generated by lithium compounds in the reaction medium, prior to removing iodides from a low energy carbonylation process to produce purified acetic acid.

BACKGROUND OF THE INVENTION

Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, incorporated herein by reference in its entirety. The carbonylation catalyst contains rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms. Likewise, because the nature of the halide promoter is not generally critical, a large number of suitable promoters, most of which are organic iodides, may be used. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.

A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalyst contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium comprises acetic acid, methyl acetate, water, methyl iodide and the catalyst. Commercial processes for the carbonylation of methanol include those described in U.S. Pat. No. 3,769,329, the entireties of which is incorporated herein by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entirety of which is incorporated herein by reference.

The AO™ process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in U.S. Pat. Nos. 5,001,259; 5,026,908; and 5,144,068; and EP0161874, the entireties of which are incorporated herein by reference. As disclosed therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate (MeAc), methyl halide, especially methyl iodide (MeI), and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at high levels, even at very low water concentrations, i.e., 4 weight percent or less, (despite the prior practice of maintaining approximately 14-15 wt. % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium, at least a finite concentration of water, e.g., 0.1 wt. %, and a specified concentration of iodide ions over and above the iodide ion that is present as hydrogen iodide. This iodide ion is a simple salt, with lithium iodide being preferred. The salt may be formed in situ, for example, by adding lithium acetate, lithium carbonate, lithium hydroxide or other lithium salts of anions compatible with the reaction medium. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, a high degree of catalyst stability and reactor productivity is achieved even when the liquid reaction medium contains water in finite concentrations as low as 0.1 wt. %. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e., resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide, which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium.

U.S. Pat. No. 5,144,068, the entirety of which is incorporated herein by reference, discloses a process for producing acetic acid by reacting methanol with carbon monoxide in a liquid reaction medium containing a rhodium (Rh) catalyst and comprising water, acetic acid, methyl iodide, and methyl acetate, wherein catalyst stability is maintained in the reaction by maintaining in said reaction medium during the course of said reaction 0.1 wt. % to 14 wt. % of water together with (a) an effective amount in the range of 2 wt. % to 20 wt. % of a catalyst stabilizer selected from the group consisting of iodide salts which are soluble in said reaction medium in effective concentration at reaction temperature, (b) 5 wt. % to 20 wt. % of methyl iodide, and (c) 0.5 wt. % to 30 wt. % of methyl acetate. Suitable iodide salts may be a quaternary iodide salt or an iodide salt of a member of the group consisting of the metals of Group IA and Group IIA of the Periodic Table.

Carbonyl impurities, such as acetaldehyde, that are formed during the carbonylation of methanol may react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, and the like. It is desirable to remove multi-carbon alkyl iodides from the reaction product because even small amounts of these impurities in the acetic acid product tend to poison the catalyst used in the production of vinyl acetate, a product commonly produced from acetic acid.

Conventional techniques to remove such impurities include treating the crude acid product streams with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like. Such treatments may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations to yield a suitable purified acetic acid as the final product. It is also known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the purified acetic acid adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.

While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the low-water carbonylation process and purification treatment described above frequently remains somewhat deficient with respect to the permanganate time due to the presence of small proportions of residual impurities. Because a sufficient permanganate time is an important commercial test, which the acid product may be required to meet to be suitable for many uses, the presence of impurities that decrease permanganate time is objectionable. Moreover, it has not been economically or commercially feasible to remove minute quantities of these impurities from the acetic acid by distillation because some of the impurities have boiling points close to that of the acetic acid product or halogen-containing catalyst promoters, such as methyl iodide. It has thus become important to identify economically viable methods of removing impurities elsewhere in the carbonylation process without contaminating the purified acetic acid or adding unnecessary costs.

Macroreticulated or macroporous strong acid cationic exchange resin compositions are conventionally utilized to reduce iodide contamination. Suitable exchange resin compositions, e.g., the individual beads thereof, comprise both sites that are functionalized with a metal, e.g., silver, mercury or palladium, and sites that remain in the acid form. Exchange resin compositions that have little or no metal-functionality do not efficiently remove iodides and, as such, are not conventionally used to do so. Typically, metal-functionalized exchange resins are provided in a fixed bed and a stream comprising the crude acetic acid product is passed through the fixed resin bed. In the metal functionalized resin bed, the iodide contaminants contained in the crude acetic acid product are removed from the crude acid product stream.

U.S. Pat. No. 6,657,078 describes a low-water process that uses a metal-functionalized exchange resin to remove iodides. The reference also avoids the use of a heavy ends column, resulting in an energy savings.

The metal-functionalization of exchange resin compositions often involves significant processing and expense, often costing orders of magnitude more than resins that are not metal-functionalized. Often the process steps associated with the functionalization varies very little with regard to the actual amount of metal that is deposited on the exchange resin. For example, the processing necessary to functionalize 50% of the active sites of a quantity of exchange resin is quite similar to the processing necessary to functionalize 10% of the active sites of the same quantity of exchange resin. Because the entire quantity of exchange resin requires processing, however, both the 50%-functionalized exchange resin and the 10%-functionalized resin require significantly more processing than the same quantity of non-functionalized resin.

Other ion exchange resins have been used to remove iodide impurities from acetic acid and/or acetic anhydride. There is disclosed in U.S. Pat. No. 5,220,058 the use of ion exchange resins having metal exchanged thiol functional groups for removing iodide impurities from acetic acid and/or acetic anhydride. Typically, the thiol functionality of the ion exchange resin has been exchanged with silver, palladium, or mercury.

In addition to iodide contaminants, metals from the walls of the vessels used in the acetic acid production system often corrode and dissolve into the crude acetic acid product compositions. Thus, conventional crude acid product streams often comprise corrosion metal contaminants as well as iodide contaminants. These corrosion metals are known to interfere with the carbonylation reaction or accelerate competing reactions such as the water-gas shift reaction. Typically, these corrosion metals may be removed from the process streams by passing the streams through resin beds comprising standard macroreticular or macroporous cationic exchange resins.

In a case where a silver, mercury or palladium exchanged resin is utilized, however, the soluble corrosion metal cations may detrimentally displace the metal-functionalized sites of the exchange resins. As such, these exchange sites are unable to capture/remove the iodide contaminants. The lifetime of the functionalized resin, with regard to iodide removal, is shortened by the presence of corrosion metals. Often a pre-determined portion of the sites of the exchange resin composition are functionalized, thus leaving the remainder of the sites in the acid form. As a result, the acid sites capture much of the corrosion metals while many of the functionalized sites remain available for iodide removal. Although this technique may improve the lifetime of exchange resins, the partial functionalization of the pre-determined portion of sites requires significant processing and resources.

In addition, it has been found that a problem associated with the use of silver-exchanged strong acid cation exchange resins is that the silver may actually be displaced by corrosion metals, as described in U.S. Pat. No. 5,344,976. According to this patent, the metal ion contaminants in the acid and/or anhydride may arise from corrosion or the use of reagents in the up stream process. The patent describes the use of a cationic exchanger in the acid form to remove at least a portion of the metal ion contaminants such as iron, potassium, calcium, magnesium, and sodium from a carboxylic acid stream prior to contacting the stream with the exchanged strong acid cation exchange resin to remove C₁ to C₁₀ alkyl iodide compounds, hydrogen iodide or iodide salts. However, this process does not describe purification for low-energy and low-water carbonylation processes as described above that may contain lithium and larger alkyl iodide compounds, in addition to iodides.

In addition, other schemes introduce other contaminants that may need to be removed from the product. For example, it has been well known in the art for some time that adding an alkali component such as KOH to the drying column of a carbonylation purification process is useful to inhibit the buildup of HI in the column. See, e.g., US Pub. No. 2013/0264186 and earlier references. However, this addition introduces a potassium cation into the process that can also displace the silver in a silver-exchanged strong acid cation exchange resin.

Other processes remove corrosion metal contaminants at different stages of the process, for example from the reactant composition. U.S. Pat. No. 4,894,477 describes a process that uses strongly acidic ion exchange resins in the lithium form to remove corrosion metal contaminants. U.S. Pat. No. 5,731,252 describes contacting the catalyst solution with an ion exchange resin bed, in the lithium form, while requiring simultaneous addition of a sufficient amount of water to allow the corrosion metal salts in the catalyst medium to dissociate so that ion exchange can occur and the corrosion metals can be removed from the reactor catalyst solution.

While the above-described processes have been successful, the need exists for process for improved processes for producing acetic acid, in particular, low water and low energy processes and methods for removing contaminants from those processes.

SUMMARY OF THE INVENTION

This invention relates to processes for the production of acetic acid. In one embodiment the present invention relates to process for producing acetic acid that involves carbonylating either methanol, dimethyl ether, methyl acetate, or some combination thereof in the presence of water in an amount from 0.1 to 14 wt. % based on the total weight, a rhodium catalyst, methyl iodide, and a lithium compound, to form a reaction medium in a reactor; then separating the reaction medium to form a liquid recycle stream and a vapor product stream; separating the vapor product stream in up to 2 distillation columns in a primary purification train to produce a crude acid product comprising acetic acid and lithium, which may be derived from and/or generated by the lithium compound in the reaction medium, then contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product; and finally contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid. The intermediate acetic acid product can comprise less lithium than the crude acetic acid product, for example, the intermediate acetic acid product can comprise, in some embodiments, less than or equal to 100 wppb lithium, based on the total weight of the intermediate acetic acid product. In some embodiments, the purified acetic acid product can comprise less than or equal to 100 wppb lithium, and/or a metal displaced from the metal-exchanged ion exchange resin, e.g., silver, mercury, palladium and rhodium, in an amount of less than or equal to 100 wppb, and/or less than or equal to 100 wppb iodides, based on the purified total weight of the acetic acid product. In some embodiments, the process can further comprise a step of adding a potassium salt selected from the group consisting of potassium acetate, potassium carbonate, and potassium hydroxide to the distilled acetic acid product prior to distilling the distilled acetic acid product in a second distillation column, wherein at least a portion of the potassium is removed by the cationic exchanger in the acid form.

In some embodiments, the process can further comprise condensing an low boiling point overhead obtained from the first distillation column to form a heavy liquid phase and a light liquid phase, and wherein a portion of the heavy liquid phase is treated to remove at least one permanganate reducing compound selected from the group consisting of acetaldehyde, acetone, methyl ethyl ketone, butylaldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and the aldol condensation products thereof.

In some embodiments, the step of separating the vapor product stream can comprise distilling the vapor product stream in a first distillation column to form a sidedraw comprising acetic acid; and then distilling the sidedraw in a second distillation column to produce a crude acid product comprising acetic acid and cations selected from the group consisting of Groups IA and IIA of the periodic table, quaternary nitrogen cations, and phosphorous-containing cations, e.g., preferably lithium cations.

In some embodiments, the crude acetic acid product is removed as a side stream from above the bottom of the second distillation column. This can be, for example, a liquid stream. In other embodiments, the crude acetic acid product can be removed as a residue from the bottom of the second distillation column.

In other embodiments, the invention relates to a process for producing acetic acid comprising carbonylating at least one member selected from the group consisting of methanol, dimethyl ether, and methyl acetate in the presence of water in an amount from 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide and iodide salts, to form a reaction medium in a reactor; separating the reaction medium to form a liquid recycle stream and a vapor product stream; separating the vapor product stream in up to 2 distillation columns in a primary purification train to produce a crude acid product comprising acetic acid comprising at least one cation selected from the group consisting of Groups IA and IIA of the periodic table, quaternary nitrogen cations, and phosphorous-containing cations, wherein the at least one cation may be derived from and/or generated by a compound in the reaction medium, contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product; and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid.

In still other embodiments, the invention relates to a process for removing iodides from a liquid composition comprising a carboxylic acid or an anhydride thereof, greater than 10 wppb of C₁₀-C₁₄ alkyl iodides, iodide anions, and a cation selected from the group consisting of Group IA and IIA metals and quaternary nitrogen cations, and quaternary phosphorous-containing cations, which process comprises contacting said liquid composition with a cationic exchanger in the acid form to produce an intermediate product with a reduced concentration of cations selected from the group consisting of Group IA and IIA metals, quaternary nitrogen cations, and phosphorous-containing cations; and contacting the intermediate product with a metal-exchanged ion exchange resin having acid cation exchange sites comprising at least one metal selected from the group consisting of silver, mercury, palladium and rhodium to produce a purified acetic acid product.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood in view of the appended non-limiting figures, wherein:

FIG. 1 illustrates a process for producing acetic acid with a metal functionalized fixed resin bed for iodide removal.

FIG. 2 illustrates another process for producing acetic acid with a metal functionalized fixed resin bed for iodide removal.

DETAILED DESCRIPTION OF THE INVENTION

At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation—specific decisions must be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. In addition, the processes disclosed herein can also comprise components other than those cited or specifically referred to, as is apparent to one having average or reasonable skill in the art.

In the summary and this detailed description, each numerical value should be read once as modified by the term “about” (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration within the range, including the end points, is to be considered as having been stated. For example, a range “from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific data points, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors possessed knowledge of the entire range and all points within the range.

Throughout the entire specification, including the claims, the following terms have the indicated meanings unless otherwise specified.

As used in the specification and claims, “near” is inclusive of “at.” The term “and/or” refers to both the inclusive “and” case and the exclusive “or” case, and is used herein for brevity. For example, a mixture comprising acetic acid and/or methyl acetate may comprise acetic acid alone, methyl acetate alone, or both acetic acid and methyl acetate.

All percentages are expressed as weight percent (wt. %), based on the total weight of the particular stream or composition present, unless otherwise noted. Room temperature is 25° C. and atmospheric pressure is 101.325 kPa unless otherwise noted.

For purposes herein: acetic acid may be abbreviated as “AcOH”;

-   -   acetaldehyde may be abbreviated as “AcH”;     -   methyl acetate may be abbreviated “MeAc”;     -   methanol may be abbreviated “MeOH”;     -   methyl iodide may be abbreviated as “MeI”;     -   hydrogen iodide may be abbreviated as “HI”;     -   carbon monoxide may be abbreviated “CO”; and     -   dimethyl ether may be abbreviated “DME”.

HI refers to either molecular hydrogen iodide or dissociated hydriodic acid when at least partially ionized in a polar medium, typically a medium comprising at least some water. Unless otherwise specified, the two are referred to interchangeably. Unless otherwise specified, HI concentration is determined via acid-base titration using a potentiometric end point. In particular, HI concentration is determined via titration with a standard lithium acetate solution to a potentiometric end point. It is to be understood that for purposes herein, the concentration of HI is not determined by subtracting a concentration of iodide assumed to be associated with a measurement of corrosion metals or other non H+ cations from the total ionic iodide present in a sample.

It is to be understood that HI concentration does not refer to iodide ion concentration. HI concentration specifically refers to HI concentration as determined via potentiometric titration.

This subtraction method is an unreliable and imprecise method to determine relatively lower HI concentrations (i.e., less than about 5 weight percent) due to the fact that it assumes all non-H+ cations (such as cations of Fe, Ni, Cr, Mo) are associated with iodide anion exclusively. In reality, a significant portion of the metal cations in this process can be associated with acetate anion. Additionally, many of these metal cations have multiple valence states, which adds even more unreliability to the assumption on the amount of iodide anion which could be associated with these metals. Ultimately, this method gives rise to an unreliable determination of the actual HI concentration, especially in view of the ability to perform a simple titration directly representative of the HI concentration.

For purposes herein, an “overhead” or “distillate” of a distillation column refers to at least one of the lower boiling condensable fractions which exits at or near the top, (e.g., proximate to the top), of the distillation column, and/or the condensed form of that stream or composition. Obviously, all fractions are ultimately condensable, yet for purposes herein, a condensable fraction is condensable under the conditions present in the process as readily understood by one of skill in the art. Examples of noncondensable fractions may include nitrogen, hydrogen, and the like. Likewise, an overhead stream may be taken just below the upper most exit of a distillation column, for example, wherein the lowest boiling fraction is a non-condensable stream or represents a de-minimis stream, as would be readily understood by one of reasonable skill in the art.

The “bottoms” or “residuum” of a distillation column refers to one or more of the highest boiling fractions which exit at or near the bottom of the distillation column, also referred to herein as flowing from the bottom sump of the column. It is to be understood that a residuum may be taken from just above the very bottom exit of a distillation column, for example, wherein the very bottom fraction produced by the column is a salt, an unusable tar, a solid waste product, or a de-minimis stream as would be readily understood by one of reasonable skill in the art.

For purposes herein, distillation columns comprise a distillation zone and a bottom sump zone. The distillation zone includes everything above the bottom sump zone, i.e., between the bottom sump zone and the top of the column. For purposes herein, the bottom sump zone refers to the lower portion of the distillation column in which a liquid reservoir of the higher boiling components is present (e.g., the bottom of a distillation column) from which the bottom or residuum stream flows upon exiting the column. The bottom sump zone may include reboilers, control equipment, and the like.

It is to be understood that the term “passages”, “flow paths”, “flow conduits”, and the like in relation to internal components of a distillation column are used interchangeably to refer to holes, tubes, channels, slits, drains, and the like, which are disposed through and/or which provide a path for liquid and/or vapor to move from one side of the internal component to the other side of the internal component. Examples of passages disposed through a structure such as a liquid distributor of a distillation column include drain holes, drain tubes, drain slits, and the like, which allow a liquid to flow through the structure from one side to another.

Average residence time is defined as the sum total of all liquid volume hold-up for a given phase within a distillation zone divided by the average flow rate of that phase through the distillation zone. The hold-up volume for a given phase can include liquid volume contained in the various internal components of the column including collectors, distributors and the like, as well as liquid contained on trays, within downcomers, and/or within structured or random packed bed sections.

Lithium Removal

This invention relates to processes for the production of acetic acid and, in particular, to improved processes for removing cations, such as lithium, and iodides, including higher molecular weight iodides, for example, C₁₀-C₁₄ alkyl iodides, from a low energy carbonylation process. The process is capable of removing at least one cation derived from and/or generated by a compound in the reaction medium and these cations may be selected from the group consisting of Groups IA and IIA of the periodic table, quaternary nitrogen cations, and phosphorous-containing cations. These cations may be derived from and/or generated by a compound in the reaction medium. In one embodiment, the process is directed to removing a lithium cation derived from and/or generated by the lithium compound in the reaction medium, such as lithium iodide or lithium acetate. According to the present invention the cation, and preferably lithium cation, is removed prior to the iodide removal to prevent displacement in the metal ion-exchange resin.

With ever increasing cost pressures and higher energy prices, there has been ever increasing motivation to simplify chemical manufacturing operations and particularly to reduce the number of manufacturing steps. In this regard, it is noted that U.S. Pat. No. 5,416,237 discloses a single zone distillation process for making acetic acid. Such process modifications, while desirable in terms of energy costs, tend to place increasing demands on the purification train. In particular, fewer recycles and distillations tend to introduce (or fail to remove) a higher level of iodides and other promoters in the crude acid product, and particularly more iodides of a higher molecular weight. For example, octyl iodide, decyl iodide and dodecyl iodides may all be present in the crude acid product as well as hexadecyl iodide; all of which are difficult to remove by conventional techniques.

Low water and low energy processes for producing acetic acid by the carbonylation of methanol have been developed which involve a rhodium-catalyzed system operating at a water concentration of less than or equal to 14 wt. %, and more preferably less than or equal to 4.1 wt. %, and utilizing up to 2 distillation columns in the primary purification train. The primary purification train is directed at removing bulk components, such as water, methyl acetate, methyl iodide, and hydrogen iodide, from the vapor product stream from the reactor/flash vessel to obtain acetic acid. This primary purification train receives the majority of the vapor flow from the reactor and obtains acetic acid as a purified acetic acid. For example, the columns of the primary purification train include the light ends column and drying column. This primary purification train may exclude columns whose main function is to remove minor components such as acetaldehyde, alkanes, and propionic acid.

The process for producing acetic acid in the reaction may use cations, such as a lithium cations, that have been found by the present inventors to collect in the crude acid product. Unlike other metals that may be present in the crude acetic acid product that are the result of corrosion metals or metals from the up stream process, i.e. adding after the reactor, lithium cations may be derived from and/or generated by the lithium compound in the reaction medium. It was previously understood that the lithium compounds were less volatile and remained in the liquid recycle from the flash vessel. It has now been discovered that lithium cations derived from and/or generated by the lithium compound in the reaction medium may be entrained or be volatile enough to concentrate with the crude acetic acid product after purification in the primary purification trains. These residual lithium cations may be difficult to remove and in the final metal-exchange guard bed may adversely replace metals in the guard bed, resulting in poor iodide removal and increased metals displaced from the metal-exchanged ion exchange resin, e.g. silver, mercury, palladium and rhodium, in the purified acetic acid. Thus, the purified acetic acid may also have unacceptable levels of iodides despite using a metal exchange guard bed. The present invention provides process for removing the cations, particularly the lithium cations.

The source of the cation may be derived from or generated by a variety promoters, co-catalysts, additives, in situ reactions, etc. For example low water and low energy processes that involve the use of a promoter such as lithium iodide, which may form in situ following the addition of lithium acetate or other compatible lithium salts to the reaction mixture. Therefore, process streams may contain some quantity of lithium ions derived from and/or generated by the lithium compound in the reaction medium. It was previously understood that lithium ions did not affect separation and purity in the primary purification train. In addition, since the process uses a maximum of 2 distillation columns in the primary purification train and preferably the primary purification does not include a column to remove heavy ends materials, the crude acid product may contain larger alkyl iodide compounds, e.g., C₁₀-C₁₄ alkyl iodides, in addition to cations, such as lithium. In some embodiments, greater than or equal to 10% of the iodides present, or even greater than or equal to 50%, have an organic chain length of more than 10 carbon atoms. Thus, the crude acid product may comprise C₁₀-C₁₄ alkyl iodides in an amount of greater than or equal to 10 weight parts per billion (wppb), e.g., greater than or equal to 20 wppb, greater than or equal to 50 wppb, greater than or equal to 100 wppb, greater than or equal to 1 wppm, or greater than or equal to 10 wppm. These higher alkyl iodides may be in addition to the usual shorter chain length iodide impurities found in the crude acid product of an iodide promoted carbonylation process, including methyl iodide, HI, and hexyl iodide. Iodide impurities are typically removed from the crude acid product using a metal-exchanged strong acid ion exchange resin in which the metal is silver or mercury, for example. However, it has been found that the silver or mercury in the metal-exchanged strong acid ion exchange resin may be displaced by the residual lithium, resulting in lower resin capacity and efficiency and the potential for contaminating the product with silver or mercury.

The cation in the crude acid product may result from the use of organic alkali salt ligands, such as organic lithium salt ligands, such as those described CN101053841 and CN1349855, the entire contents and disclosure of which are hereby incorporated by reference. CN101053841 describes a ligand comprising lithium acetate or lithium oxalate. CN1349855 describes a bimetallic catalyst having a metal lithium organic ligand coordinating cis-dicarbonyl rhodium structure. The metal lithium organic ligand may be a pyridine derivative, such as lithium pyridine-2-formate, lithium pyridine-3-formate, lithium pyridine-4-formate, lithium pyridine-3-acetate, lithium pyridine-4-acetate, or lithium pyridine-3-propionate. In fact, the lithium salt component of all of these ligands are believed to generate lithium iodide in situ within the reaction medium after exposure to methyl iodide at reaction temperatures and pressures in the carbonylation reactor. At least some small portion of the lithium component may entrain into the purification system. Thus, the present invention may also remove lithium formed in situ from use of these types of organic ligands.

Cations may also be present as a result of the use of non-lithium salts, such as through the use of bimetallic Rh chelating catalysts that have an amine functionality, such as those described in CN1640543, the entire contents and disclosure of which is hereby incorporated by reference. According to CN16040543 the cation species contains N and O donor atoms and is formed from aminobenzoic acid. The amine may quaternize with methyl iodide in situ within the reaction medium at reaction temperature and pressure to form a quaternary nitrogen cation. The quaternary nitrogen cation, similar to the lithium cation, may be carried through with the crude acid product and may be removed using the present invention prior to the metal-exchange guard beds.

The present invention therefore involves a low water and low energy process for producing acetic acid by the carbonylation of methanol, dimethyl ether, and/or methyl acetate in the presence of a water concentration from 0.1 to 14 wt. %, a metal catalyst, methyl iodide and lithium compounds, such as lithium acetate or lithium iodide. The invention utilizes up to 2 distillation columns in the primary purification train and purifies the resulting acidic acid product with a cationic exchanger in the acid form to remove residual lithium ions derived from and/or generated by the lithium compound in the reaction medium followed by treatment with a metal-exchanged ion exchange resin having acid cation exchange sites comprising at least one metal selected from the group consisting of silver, mercury, palladium and rhodium. The metal-exchanged ion exchange resin can have at least 1% of the strong acid exchange sites occupied by silver, mercury, palladium, and/or rhodium, e.g., at least 2% silver, mercury, palladium, and/or rhodium, at least 5% silver, mercury, palladium, and/or rhodium, at least 10% silver, mercury, palladium, and/or rhodium, or at least 20% silver, mercury, palladium, and/or rhodium. By using a cationic exchanger to remove lithium prior to the use of a resin having metal-exchanged strong acid cation sites, the displacement of silver, mercury, palladium and/or rhodium from the metal-exchanged sites by the lithium is reduced or eliminated for a process that utilizes up to 2 distillation columns in the primary purification train.

Particularly preferred processes are those utilizing a cationic exchanger for removing lithium followed by a silver-exchanged cationic substrate for removing iodides. The crude acid product in many cases includes organic iodides with a C₁₀ or more aliphatic chain length which need to be removed. Sometimes more than 10% of the iodides present, e.g., more than 15%, more than 30% or even more than 50%, have an organic chain length of 10 carbon atoms or more.

Decyl iodides and dodecyl iodides are especially prevalent in the absence of heavy ends and other finishing apparatus and are difficult to remove from the product. The silver-exchanged cationic substrates of the present invention typically remove over 90% of such iodides; oftentimes the crude acid product has from 10 to 1000 wppb total iodide prior to treatment which would make the product unusable for iodide-sensitive applications.

An iodide level of 20 wppb to 1.5 wppm in the crude acid product prior to iodide removal treatment is typical; whereas the iodide removal treatment is preferably operative to remove at least about 95% of the total iodide present. In a typical embodiment, lithium/iodide removal treatment involves contacting the crude acid product with a cationic exchanger to remove 95 wt. % or more, e.g., 95 wt. % or more, 97 wt. % or more, 98 wt. % or more, 99 wt. % or more, or 99.5 wt. % or more of the lithium ions followed by contacting the crude acid product with a silver-exchanged sulfonic acid functionalized macroreticular ion exchange resin, wherein the product has an organic iodide content of greater than 100 wppb, e.g., greater than 100 wppb, greater than 200 wppb, greater than 400 wppb, greater than 500 wppb, or greater than 1000 wppb, prior to treatment and an organic iodide content of less than 10 wppb, e.g., less than 10 wppb, less than 7 wppb, less than 5 wppb, less than 3 wppb, less than 2 wppb, less than 1 wppb, after contacting the resin.

Lithium has also been found to be entrained in the crude acid product in the absence of heavy ends and other finishing apparatus. Even in very small amounts of 10 wppb of lithium in the crude acid product may cause problem for removing iodides. The lithium in the acid-containing crude acid product exiting the drying column of an acetic acid process, e.g., the last column in the primary purification train, may be in an amount up to or equal to 10 wppm, e.g., up to or equal to 5 wppm, up to or equal to 1 wppm, up to or equal to 500 wppb, up to or equal to 300 wppb, or up to or equal to 100 wppb. In terms of ranges, the crude acid product may comprise lithium in an amount from 0.01 wppm to 10 wppm, e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. By utilizing a cationic exchanger in the acid form before introducing the crude acid product to a metal-exchanged resin, significant amounts of lithium can be removed. For example greater than or equal to 90 wt. % of the lithium in the stream might be removed by the cationic exchanger, e.g., greater than 92, wt. %, greater than 95 wt. %, greater than 98 wt. %, or greater than 99 wt. %. Thus, the stream exiting the acid-form cationic exchanger may contain less than or equal to 50 wppb lithium, e.g., less than 25 wppb lithium, less than or equal to 10 wppb, or less than or equal to 5 wppb. Such removal of the lithium can greatly extend the life of the metal-exchanged resin.

Acetic Acid Production Systems

An exemplary acetic acid production process is described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The purification processes described herein may be useful in carbonylation processes that use methanol and/or methyl acetate (MeAc), methyl formate or dimethyl ether, or mixtures thereof, to produce acetic acid in the presence of a Group VIII metal catalyst, such as rhodium, and a halogen-containing catalyst promoter. A particularly useful process is the low water rhodium-catalyzed carbonylation of methanol to acetic acid as exemplified in U.S. Pat. No. 5,001,259. Other metal catalysts, e.g., iridium-based catalysts, are contemplated as well.

Generally, the metal component, e.g., rhodium component, of the catalyst system is believed to be present in the form of a coordination compound of rhodium with a halogen component providing at least one of the ligands of such coordination compound. In addition to the coordination of rhodium and halogen, it is also believed that carbon monoxide coordinates with rhodium. The rhodium component of the catalyst system may be provided by introducing into the reaction zone rhodium in the form of rhodium metal, rhodium salts such as the oxides, acetates, iodides, carbonates, hydroxides, chlorides, etc., or other compounds that result in the formation of a coordination compound of rhodium in the reaction environment.

The metal catalyst may comprise a Group VIII metal. Suitable Group VIII catalysts include rhodium and/or iridium catalysts. When a rhodium catalyst is used, the rhodium catalyst may be added in any suitable form such that rhodium is in the catalyst solution as an equilibrium mixture including [Rh(CO)₂I₂]-anion, as is well known in the art. Iodide salts optionally maintained in the reaction mixtures of the processes described herein may be in the form of a soluble salt of an alkali metal or alkaline earth metal, quaternary ammonium, phosphonium salt or mixtures thereof. In certain embodiments, the catalyst co-promoter is lithium iodide, lithium acetate, or mixtures thereof. The salt co-promoter may be added as a non-iodide salt that generates an iodide salt. The iodide catalyst stabilizer may be introduced directly into the reaction system. Alternatively, the iodide salt may be generated in-situ since under the operating conditions of the reaction system, a wide range of non-iodide salt precursors reacts with methyl iodide or hydroiodic acid in the reaction medium to generate the corresponding co-promoter iodide salt stabilizer. For additional detail regarding rhodium catalysis and iodide salt generation, see U.S. Pat. Nos. 5,001,259; 5,026,908; 5,144,068 and 7,005,541, the entireties of which are hereby incorporated by reference. The carbonylation of methanol utilizing iridium catalyst is well known and is generally described in U.S. Pat. Nos. 5,942,460, 5,932,764, 5,883,295, 5,877,348, 5,877,347 and 5,696,284, the entireties of which are hereby incorporated by reference.

The halogen-containing catalyst promoter of the catalyst system consists of a halogen compound comprising an organic halide. Thus, alkyl, aryl, and substituted alkyl or aryl halides can be used. Preferably, the halogen-containing catalyst promoter is present in the form of an alkyl halide. Even more preferably, the halogen-containing catalyst promoter is present in the form of an alkyl halide in which the alkyl radical corresponds to the alkyl radical of the feed alcohol, which is being carbonylated. Thus, in the carbonylation of methanol to acetic acid, the halide promoter may include methyl halide, and more preferably methyl iodide.

The components of the reaction medium are maintained within defined limits to ensure sufficient production of acetic acid. The reaction medium contains a concentration of the metal catalyst, e.g., rhodium catalyst, in an amount from 200 to 3000 wppm, e.g., from 500 to 2000 wppm, or from 600 to 1500 wppm. The concentration of water in the reaction medium is maintained under low water conditions, e.g., water in amount of less than or equal to 14 wt. %, from 0.1 wt. % to 14 wt. %, from 0.2 wt. % to 10 wt. % or most preferably from 0.25 wt. % to 5 wt. %. The concentration of methyl iodide in the reaction medium is maintained to be from 1 to 25 wt. %, e.g., from 5 to 20 wt. %, from 4 to 13.9 wt. %. The concentration of iodide salt, e.g., lithium iodide, in the reaction medium is maintained to be from 1 to 25 wt. %, e.g., from 2 to 20 wt. %, from 3 to 20 wt. %. The concentration of methyl acetate in the reaction medium is maintained to be from 0.5 to 30 wt. %, e.g., from 0.3 to 20 wt. %, from 0.6 to 4.1 wt. %. The following amounts are based on the total weight of the reaction medium. The ranges disclosed in this application include the endpoints, subranges and individual values unless otherwise stated.

The concentration of acetic acid in the reaction medium is generally greater than or equal to 30 wt. %, e.g., greater than or equal to 40 wt. %, greater than or equal to 50 wt. %, or greater than or equal to 60 wt. %.

In embodiments, the process for producing acetic acid further includes introducing a lithium compound into the reactor to maintain the concentration of lithium acetate in an amount from 0.3 to 0.7 wt. % in the reaction medium, wherein in an exemplary embodiment, in the reaction medium the concentration of the rhodium catalyst is maintained in an amount from 200 to 3000 wppm in the reaction medium, the concentration of water is maintained in amount from 0.1 to 4.1 wt. % in the reaction medium, and the concentration of methyl acetate is maintained from 0.6 to 4.1 wt. % in the reaction medium, based on the total weight of the reaction medium present within the carbonylation reactor.

In embodiments, the lithium compound introduced into the reactor is selected from the group consisting of lithium acetate, lithium carboxylates, lithium carbonates, lithium hydroxide, other organic lithium salts, and mixtures thereof. In embodiments, the lithium compound is soluble in the reaction medium. In an embodiment, lithium acetate dihydrate may be used as the source of the lithium compound.

Lithium acetate reacts with hydrogen iodide according to the following equilibrium reaction (I) to form lithium iodide and acetic acid:

LiOAc+HI⇄LiI+HOAc  (I)

Lithium acetate is thought to provide improved control of hydrogen iodide concentration relative to other acetates, such as methyl acetate, present in the reaction medium. Without being bound by theory, lithium acetate is a conjugate base of acetic acid and thus reactive toward hydrogen iodide via an acid-base reaction. This property is thought to result in an equilibrium of the reaction (I) which favors reaction products over and above that produced by the corresponding equilibrium of methyl acetate and hydrogen iodide. This improved equilibrium is favored by water concentrations of less than 4.1 wt. % in the reaction medium. In addition, the relatively low volatility of lithium acetate compared to methyl acetate allows the lithium acetate to remain in the reaction medium except for volatility losses and small amounts of entrainment into the vapor crude product. In contrast, the relatively high volatility of methyl acetate allows the material to distill into the purification train, rendering methyl acetate more difficult to control. Lithium acetate is much easier to maintain and control in the process at consistent low concentrations of hydrogen iodide. Accordingly, a relatively small amount of lithium acetate may be employed relative to the amount of methyl acetate needed to control hydrogen iodide concentrations in the reaction medium. It has further been discovered that lithium acetate is at least three times more effective than methyl acetate in promoting methyl iodide oxidative addition to the rhodium [I] complex. However, it has been discovered that lithium cations derived from and/or generated by the lithium compound in the reaction medium may be entrained or be volatile enough to concentrate with the crude acetic acid product after purification in the primary purification trains.

In embodiments, the concentration of lithium acetate in the reaction medium is maintained at greater than or equal to 0.3 wt. %, or greater than or equal to 0.35 wt. %, or greater than or equal to 0.4 wt. %, or greater than or equal to 0.45 wt. %, or greater than or equal to 0.5 wt. %, and/or in embodiments, the concentration of lithium acetate in the reaction medium is maintained at less than or equal to 0.7 wt. %, or less than or equal to 0.65 wt. %, or less than or equal to 0.6 wt. %, or less than or equal to 0.55 wt. %.

It has been discovered that an excess of lithium acetate in the reaction medium can adversely affect the other compounds in the reaction medium, leading to decrease productivity. Conversely, it has been discovered that a lithium acetate concentration in the reaction medium below about 0.3 wt. % is unable to maintain the desired hydrogen iodide concentrations in the reaction medium of below 1.3 wt. %.

In embodiments, the lithium compound may be introduced continuously or intermittently into the reaction medium. In embodiments, the lithium compound is introduced during reactor start up. In embodiments, the lithium compound is introduced intermittently to replace entrainment losses.

Thus, in one embodiment there is provided a process for producing acetic acid comprising carbonylating at least one member selected from the group consisting of methanol, dimethyl ether, and methyl acetate in the presence of water in an amount from 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide and a lithium iodide, to form a reaction medium in a reactor, wherein the concentration of lithium acetate in the reaction medium is maintained at less than or equal to 0.7 wt. %, separating the reaction medium to form a liquid recycle stream and a vapor product stream, separating the vapor product stream in up to 2 distillation columns in a primary purification train to produce a crude acid product comprising acetic acid and at least one lithium cation derived from and/or generated by the lithium compound in the reaction medium, contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product, and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid.

In some embodiments, the desired reaction rates are obtained even at low water concentrations by maintaining in the reaction medium an ester of the desired carboxylic acid and an alcohol, desirably the alcohol used in the carbonylation, and an additional iodide ion that is over and above the iodide ion that is present as hydrogen iodide. A desired ester is methyl acetate. The additional iodide ion is desirably an iodide salt, with lithium iodide (LiI) being preferred. It has been found, as described in U.S. Pat. No. 5,001,259, that under low water concentrations, methyl acetate and lithium iodide act as rate promoters.

The carbonylation reaction of methanol to acetic acid product may be carried out by contacting the methanol feed with gaseous carbon monoxide bubbled through an acetic acid solvent reaction medium containing the rhodium catalyst, methyl iodide promoter, methyl acetate, and additional soluble iodide salt, at conditions of temperature and pressure suitable to form the carbonylation product. It will be generally recognized that it is the concentration of iodide ion in the catalyst system that is important and not the cation associated with the iodide, and that at a given molar concentration of iodide the nature of the cation is not as significant as the effect of the iodide concentration. Any metal iodide salt, or any iodide salt of any organic cation, or other cations such as those based on amine or phosphine compounds (optionally, ternary or quaternary cations), can be maintained in the reaction medium provided that the salt is sufficiently soluble in the reaction medium to provide the desired level of the iodide. When the iodide is a metal salt, preferably it is an iodide salt of a member of the group consisting of the metals of Group IA and Group IIA of the periodic table as set forth in the “Handbook of Chemistry and Physics” published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). In particular, alkali metal iodides are useful, with lithium iodide being particularly suitable. In the low water carbonylation process, the additional iodide ion over and above the iodide ion present as hydrogen iodide is generally present in the catalyst solution in amounts such that the total iodide ion concentration is from 1 to 25 wt. % and the methyl acetate is generally present in amounts from 0.5 to 30 wt. %, and the methyl iodide is generally present in amounts from 1 to 25 wt. %. The rhodium catalyst is generally present in amounts from 200 to 3000 wppm.

The reaction medium may also contain impurities that should be controlled to avoid byproduct formation. One impurity in the reaction medium may be ethyl iodide, which is difficult to separate from acetic acid. Applicant has further discovered that the formation of ethyl iodide may be affected by numerous variables, including the concentration of acetaldehyde, ethyl acetate, methyl acetate and methyl iodide in the reaction medium. Additionally, ethanol content in the methanol source, hydrogen partial pressure and hydrogen content in the carbon monoxide source have been discovered to affect ethyl iodide concentration in the reaction medium and, consequently, propionic acid concentration in the final acetic acid product.

In embodiments, the propionic acid concentration in the acetic acid product may further be maintained below 250 wppm by maintaining the ethyl iodide concentration in the reaction medium at less than or equal to 750 wppm without removing propionic acid from the acetic acid product.

In embodiments, the ethyl iodide concentration in the reaction medium and propionic acid in the acetic acid product may be present in a weight ratio from 3:1 to 1:2. In embodiments, the acetaldehyde:ethyl iodide concentration in the reaction medium is maintained at a weight ratio from 2:1 to 20:1.

In embodiments, the ethyl iodide concentration in the reaction medium may be maintained by controlling at least one of the hydrogen partial pressure, the methyl acetate concentration, the methyl iodide concentration, and/or the acetaldehyde concentration in the reaction medium.

In embodiments, the concentration of ethyl iodide in the reaction medium is maintained/controlled to be less than or equal to 750 wppm, or e.g., less than or equal to 650 wppm, or less than or equal to 550 wppm, or less than or equal to 450 wppm, or less than or equal to 350 wppm. In embodiments, the concentration of ethyl iodide in the reaction medium is maintained/controlled at greater than or equal to 1 wppm, or e.g., 5 wppm, or 10 wppm, or 20 wppm, or 25 wppm, and less than or equal to 650 wppm, or e.g., 550 wppm, or 450 wppm, or 350 wppm.

In embodiments, the weight ratio of ethyl iodide in the reaction medium to propionic acid in the acetic acid product may range from 3:1 to 1:2, or e.g., from 5:2 to 1:2, or from 2:1 to 1:2, or from 3:2 to 1:2.

In embodiments, the weight ratio of acetaldehyde to ethyl iodide in the reaction medium may range from 20:1 to 2:1, or e.g., from 15:1 to 2:1, from 9:1 to 2:1, or from 6:1.

Typical reaction temperatures for carbonylation may be from 150 to 250° C., e.g., 160 to 240° C., 170-230° C. with the temperature range of 180 to 225° C. being a preferred range. The carbon monoxide partial pressure in the reactor can vary widely but is typically from 2 to 30 atm, e.g., from 3 to 10 atm. The hydrogen partial pressure in the reactor is typically from 0.05 to 2 atm, e.g., from 1 to 1.9 atm. In some embodiments, the present invention may be operated with a hydrogen partial pressure from 0.3 to 2 atm, e.g., from 0.3 to 1.5 atm, or from 0.4 to 1.5 atm. Because of the partial pressure of by-products and the vapor pressure of the contained liquids, the total reactor pressure may range from 15 to 40 atm. The production rate of acetic acid may be from 5 to 50 mol/L·h, e.g., from 10 to 40 mol/L·h, and preferably 15 to 35 mol/L·h.

Exemplary reaction and acetic acid recovery is shown in FIG. 1. As shown, methanol-containing feed stream 101 and carbon monoxide-containing feed stream 102 are directed to liquid phase carbonylation reactor 104, in which the carbonylation reaction occurs to form acetic acid.

Carbonylation reactor 104 is preferably either a stirred vessel or bubble-column type vessel, with or without an agitator, within which the reacting liquid or slurry contents are maintained, preferably automatically, a predetermined level, which preferably remains substantially constant during normal operation. Into carbonylation reactor 104, fresh methanol, carbon monoxide, and sufficient water are continuously introduced as needed to maintain suitable concentrations in the reaction medium.

In a typical carbonylation process, carbon monoxide is continuously introduced into the carbonylation reactor, desirably below the agitator, which may be used to stir the contents. The gaseous feed preferably is thoroughly dispersed through the reacting liquid by this stirring means. Gaseous purge stream 106 desirably is vented from the reactor 104 to prevent buildup of gaseous by-products and to maintain a set carbon monoxide partial pressure at a given total reactor pressure. In one embodiment, the gaseous purge stream 106 contains low amounts of hydrogen iodide of less than or equal to 1 wt. %, e.g., less than or equal to 0.9 wt. %, less than or equal to 0.8 wt. %, less than or equal to 0.7 wt. %, less than or equal to 0.5 wt. %, less than or equal to 0.3 wt. %. Hydrogen iodide in excess of these amounts may increase the duty on the scrubber to prevent hydrogen iodide from being purged. The temperature of the reactor may be controlled and the carbon monoxide feed is introduced at a rate sufficient to maintain the desired total reactor pressure. Stream 113 comprising the liquid reaction medium exits reactor 104.

The acetic acid production system preferably includes primary purification train 108 employed to recover the acetic acid and recycle catalyst solution, methyl iodide, methyl acetate, and other system components within the process. Primary purification train 108 include light ends column 124 and drying column 130, and the associated pumps, overhead receivers, condensers, etc. Thus, a recycled catalyst solution, such as stream 110 from flash vessel 112, and optionally one or more of recycle streams 114, 116, 118, and 120, also are introduced into the reactor 104. Of course, one or more of the recycle streams may be combined prior to being introduced into the reactor. The separation system also preferably controls water and acetic acid content in the carbonylation reactor, as well as throughout the system, and facilitates PRC removal.

Flash Vessel

The reaction medium is drawn off from the carbonylation reactor 104 at a rate sufficient to maintain a constant level therein and is provided to flash vessel 112 via stream 113. In flash vessel 112, the crude product is separated in a flash separation step to obtain a vapor product stream 122 comprising acetic acid and less volatile stream 110, e.g., a liquid recycle stream, comprising a catalyst-containing solution (predominantly acetic acid containing the rhodium and the iodide salt along with lesser quantities of methyl acetate, methyl iodide, and water), which preferably is recycled to the reactor, as discussed above. The vapor product stream 122 also comprises methyl iodide, methyl acetate, water, and permanganate reducing compounds (PRC's). Dissolved gases exiting the reactor and entering the flash vessel comprise a portion of the carbon monoxide and may also contain gaseous by-products such as methane, hydrogen, and carbon dioxide. Such dissolved gases exit the flash vessel as part of the overhead stream.

In one embodiment, vapor product stream 122 comprises acetic acid, methyl iodide, methyl acetate, water, acetaldehyde, and hydrogen iodide. In one embodiment, vapor product stream 122 comprises acetic acid in an amount from 45 to 75 wt. %, methyl iodide in an amount from 20 to 50 wt. %, methyl acetate in an amount of less than or equal to 9 wt. %, and water in an amount of less than or equal to 15 wt. %, based on the total weight of the vapor product stream. In another embodiment, vapor product stream 122 comprises acetic acid in an amount from 45 to 75 wt. %, methyl iodide in an amount from 24 to less than 36 wt. %, methyl acetate in an amount of less than or equal to 9 wt. %, and water in an amount of less than or equal to 15 wt. %, based on the total weight of the vapor product stream. More preferably, vapor product stream 122 comprises acetic acid in an amount from 55 to 75 wt. %, methyl iodide in an amount from 24 to 35 wt. %, methyl acetate in an amount from 0.5 to 8 wt. %, and water in an amount from 0.5 to 14 wt. %. In yet a further preferred embodiment, vapor product stream 112 comprises acetic acid in an amount from 60 to 70 wt. %, methyl iodide in an amount from 25 to 35 wt. %, methyl acetate in an amount from 0.5 to 6.5 wt. %, and water in an amount from 1 to 8 wt. %. The acetaldehyde concentration in the vapor product stream may be in an amount from 0.005 to 1 wt. %, based on the total weight of the vapor product stream, e.g., from 0.01 to 0.8 wt. %, or from 0.01 to 0.7 wt. %. In some embodiments the acetaldehyde may be present in amounts less than or equal to 0.01 wt. %. Vapor product stream 122 may comprise hydrogen iodide in an amount less than or equal to 1 wt. %, based on the total weight of the vapor product stream, e.g., less than or equal to 0.5 wt. %, or less than or equal to 0.1 wt. %. Vapor product stream 122 is preferably substantially free of, i.e., contains less than or equal to 0.0001 wt. %, propionic acid, based on the total weight of the vapor product stream.

Less volatile stream 110 comprises acetic acid, the metal catalyst, corrosion metals, as well as other various compounds. In one embodiment, liquid recycle stream comprises acetic acid in an amount from 60 to 90 wt. %, metal catalyst in an amount from 0.01 to 0.5 wt. %; corrosion metals (e.g., nickel, iron and chromium) in a total amount from 10 to 2500 wppm; lithium iodide in an amount from 5 to 20 wt. %; methyl iodide in an amount from 0.5 to 5 wt. %; methyl acetate in an amount from 0.1 to 5 wt. %; water in an amount from 0.1 to 8 wt. %; acetaldehyde in an amount of less than or equal to 1 wt. % (e.g., from 0.0001 to 1 wt. % acetaldehyde); and hydrogen iodide in an amount of less than or equal to 0.5 wt. % (e.g., from 0.0001 to 0.5 wt. % hydrogen iodide).

Recovery of Acetic Acid

The distillation and recovery of acetic acid is not particularly limited for the purposes of the present invention. In one exemplary embodiment, there is provided a process for producing acetic acid comprising separating a reaction medium formed in a reactor in a flash vessel to form a liquid recycle and a vapor product stream, distilling the vapor product stream in a first column to obtain a side stream and a low boiling overhead vapor stream comprising water in an amount of greater than or equal to 5 wt. %, condensing the low boiling overhead vapor stream and biphasically separating the condensed stream to form a heavy liquid phase and a light liquid phase, optionally treating a portion of the heavy liquid phase and/or the light liquid phase to remove at least one PRC, distilling the side stream in a second column to obtain a crude acetic acid product in the bottoms or a liquid side stream from the second removed within 5 trays from the bottom of the second column; contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product; and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid. Various embodiments of primary purification train having up to 2 distillation columns is further described herein.

First Column

The overhead stream from flash vessel 112 is directed to the light ends column 124 as vapor product stream 122, where distillation yields a low-boiling overhead vapor stream 126, a sidedraw 128 that contains acetic acid, and a high boiling residue stream 116. In one embodiment, vapor product stream 122 may comprise acetic acid, methyl acetate, water, methyl iodide, and acetaldehyde, along with other impurities such as hydrogen iodide and crotonaldehyde, and byproducts such as propionic acid. Acetic acid removed via sidedraw 128 preferably is subjected to further purification, such as in drying column 130 for selective separation of acetic acid from water.

Light ends column 124 also preferably forms residuum or bottoms stream 116, which comprises primarily acetic acid and water. Since light ends bottoms stream 116 typically comprises some residual catalyst, it may be beneficial to recycle all or a portion of light ends bottoms stream 116 to reactor 104. Optionally, light ends bottoms stream 116 may be combined with the catalyst phase 110 from flash vessel 112 and returned together to reactor 104, as shown in FIG. 1. Although the concentration of acetic acid may be relatively high in high boiling residue stream 116, the mass flow of the high boiling residue stream 116 relative to side stream 128 is very small. In embodiments, the mass flow of the boiling residue stream 116 is less than or equal to 0.75% of side stream 128, e.g., less than or equal to 0.55%, or less than or equal to 0.45%.

In one embodiment, low-boiling overhead vapor stream 126 comprises water in amount greater than or equal to 5 wt. %, e.g., greater than or equal to 10 wt. %, or greater than or equal to 25 wt. %. The amount of water may be up to 80 wt. %. In terms of ranges, water concentration in the overhead may be from 5 wt. % to 80 wt. %, e.g., from 10 wt. % to 70 wt. % or from 25 wt. % to 60 wt. %. Reducing water concentration to less than 5 wt. % is not advantageous because this results in a large recycle of acetic acid back to the reaction system which then sets up a large recycle through the entire purification system. In addition to water, low-boiling overhead vapor stream 126 may also comprise methyl acetate, methyl iodide, and carbonyl impurities, which are preferably concentrated in the overhead to be removed from acetic acid in side stream 128. These carbonyl impurities may also be referred to herein as PRC's.

As shown, low-boiling overhead vapor stream 126 preferably is condensed and directed to an overhead phase separation unit, as shown by overhead decanter 134. Conditions are desirably maintained such that the condensed low-boiling overhead vapor stream 126, once in decanter 134, may separate to form a light liquid phase 138 and a heavy liquid phase 118. The phase separation should be maintain two separate phase, without forming a third phase or emulsion between the phases. An offgas component may be vented via line 136 from decanter 134. In embodiments, the average residence time of the condensed low-boiling overhead vapor stream 126 in overhead decanter 134 is greater than or equal to 1 minute, e.g., greater than or equal to 3 minutes, greater than or equal to 5 minutes, greater than or equal to 10 minutes, and/or the average residence time is less than or equal to 60 minutes, e.g., less than or equal to 45 minutes, or less than or equal to 30 minutes, or less than or equal to 25 minutes.

Although the specific compositions of the light phase stream 138 may vary widely, some preferred compositions are provided below in Table 1.

TABLE 1 Exemplary Light Liquid Phase from Light Ends Overhead conc. (Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-40 1-25 5-15 Water 50-90  50-80  60-80  PRC's  <5 <3 <1 MeI <10 <5 <3 MeAc 1-50 1-25 1-15

In one embodiment, overhead decanter 134 is arranged and constructed to maintain a low interface level to prevent an excess hold up of methyl iodide. Although the specific compositions of heavy liquid phase 118 may vary widely, some exemplary compositions are provided below in Table 2.

TABLE 2 Exemplary Heavy Liquid Phase from Light Ends Overhead conc. (Wt. %) conc. (Wt. %) conc. (Wt. %) Water 0.01-2  0.05-1   0.1-0.9 Methyl Acetate 0.1-25 0.5-20  0.7-15  Acetic Acid 0.1-10 0.2-8   0.5-6   PRC's <5 <3 <1 Methyl Iodide  40-98 50-95 60-85

The density of the heavy liquid phase 118 may be from 1.3 to 2, e.g., from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As described in U.S. Pat. No. 6,677,480, the measured density in the heavy liquid phase 118 correlates with the methyl acetate concentration in the reaction medium. As density decreases, the methyl acetate concentration in the reaction medium increases. In one embodiment of the present invention, heavy liquid phase 118 is recycled to the reactor and the light liquid phase 138 is controlled to be recycled through the same pump. It may be desirable to recycle a portion of the light liquid phase 138 that does not disrupt the pump and maintains a density of the combined light liquid phase 138 and heavy liquid phase of greater than or equal to 1.3, e.g., greater than or equal to 1.4, greater than or equal to 1.5, or greater than or equal to 1.7. As described herein, a portion of the heavy liquid phase 118 may be treated to remove impurities such as acetaldehyde.

As shown in FIG. 1, the light phase exits decanter 134 via stream 138. A first portion, e.g., aliquot portion, of light phase stream 138 is recycled to the top of the light ends column 124 as reflux stream 140. In other embodiments a portion of the heavy liquid phase 118 may also be refluxed (not shown) to the light ends column 124.

PRC Removal System

As described herein the light ends column 124 is part of the primary purification train. In some embodiments, a portion of light liquid phase and/or heavy liquid phase may be separated and directed to acetaldehyde or PRC removal system 132 to recover methyl iodide and methyl acetate, while removing acetaldehyde. For purposes of the present invention, the acetaldehyde or PRC removal system 132 is not part of the primary purification train.

As shown in Tables 1 and 2, light liquid phase 133 and/or heavy liquid phase 118 each contain PRC's and the process may include removing carbonyl impurities, such as acetaldehyde, that deteriorate the quality of the acetic acid product and may be removed in suitable impurity removal columns and absorbers as described in U.S. Pat. Nos. 6,143,930; 6,339,171; 7,223,883; 7,223,886; 7,855,306; 7,884,237; 8,889,904; and US Pub. Nos. 2006/0011462, which are incorporated herein by reference in their entirety. Carbonyl impurities, such as acetaldehyde, may react with iodide catalyst promoters to form alkyl iodides, e.g., ethyl iodide, propyl iodide, butyl iodide, pentyl iodide, hexyl iodide, etc. Also, because many impurities originate with acetaldehyde, it is desirable to remove carbonyl impurities from the liquid light phase.

The portion of light liquid phase 138 and/or heavy liquid phase 118 fed to the acetaldehyde or PRC removal system 132 via lines 142 and 143, respectively, may vary from 1% to 99% of the mass flow of either the light liquid phase 138 and/or heavy liquid phase 118, e.g., from 1 to 50%, from 2 to 45%, from 5 to 40%, 5 to 30% or 5 to 20%. Also in some embodiments, a portion of both the light liquid phase 138 and heavy liquid phase 118 may be fed to the acetaldehyde or PRC removal system 132. The portion of the light liquid phase 138 not fed to the acetaldehyde or PRC removal system 132 may be refluxed to the first column or recycled to the reactor, as described herein. The portion of the heavy liquid phase 118 not fed to the acetaldehyde or PRC removal system 132 may be recycled to the reactor. Although a portion of heavy liquid phase 118 may be refluxed to the light ends column, it is more desirable to return the methyl iodide enriched heavy liquid phase 118 to the reactor.

In one embodiment, a portion of light liquid phase 138 and/or heavy liquid phase 118 is fed to a distillation column which enriches the overhead thereof to have acetaldehyde and methyl iodide. Depending on the configuration, there may be two separate distillation columns, and the overhead of the second column may be enriched in acetaldehyde and methyl iodide. Dimethyl ether, which may be formed in-situ, may also be present in the overhead. The overhead may be subject to one or more extraction stages to remove a raffinate enriched in methyl iodide and an extractant. A portion of the raffinate may be returned to the distillation column, first column, overhead decanter and/or reactor. For example, when the heavy liquid phase 118 is treated in the PRC removal system 132, it may be desirable to return a portion the raffinate to either the distillation column or reactor. Also, for example, when light liquid phase 138 is treated in the PRC removal system 132, it may be desirable to return a portion the raffinate to either the first column, overhead decanter, or reactor. In some embodiments, the extractant may be further distilled to remove water, which is returned to the one or more extraction stages. The column bottoms, which contains more methyl acetate and methyl iodide than light liquid phase 138, may also be recycled to reactor 104 and/or refluxed to light ends column 124.

Drying Column

Returning to the primary purification train, in addition to the overhead phase, the light ends column 124 also forms an acetic acid sidedraw 128, which preferably comprises primarily acetic acid and water. In order to maintain an efficient product separation, it is important that the composition of the sidedraw 128 does not vary or fluctuate significantly during normal operation. By does not vary or fluctuate significantly it is meant that the concentration of the one or more C₁-C₁₄ alkyl iodides and the concentration of methyl acetate is ±0.9% of the water concentration in the side stream, e.g., ±0.7%, ±0.5%, ±0.4%, ±0.3%, ±0.2%, or ±0.1%. The water concentration in the side stream may be from 1 to 3 wt. %, e.g., preferably from 1.1 to 2.5 wt. %. For example, when the water concentration is 2.5 wt. %, the concentration of C₁-C₁₄ alkyl iodides is from 1.6 to 3.4 wt. %, and the concentration of methyl acetate is from 1.6 to 3.4 wt. %.

Optionally, a portion of the sidedraw 128 may be recirculated to the light ends column, preferably to a point below where sidedraw 128 was removed from light ends column, in order to improve the separation (not shown).

Since sidedraw 128 contains water in addition to acetic acid, sidedraw 128 from the light ends column 124 preferably is directed to drying column 130, in which the acetic acid and water are separated from one another. As shown, drying column 130, separates acetic acid sidedraw 128 to form overhead stream 144 comprised primarily of water and bottoms stream 146 comprised primarily of acetic acid. Overhead stream 144 preferably is cooled and condensed in a phase separation unit, e.g., decanter 148, to form a light phase 150 and a heavy phase 122. As shown, a portion of the light phase is refluxed, as shown by stream 152 and the remainder of the light phase is returned to the reactor 104, as shown by stream 120. The heavy phase, which typically is an emulsion comprising water and methyl iodide, preferably is returned in its entirety to the reactor 104, as shown by stream 122, optionally after being combined with stream 120.

Exemplary compositions for the light phase of the drying column overhead are provided below in Table 3.

TABLE 3 Exemplary Light Phase Compositions from Drying Column Overhead conc. (Wt. %) conc. (Wt. %) conc. (Wt. %) HOAc 1-20 1-15 1-10 Water 50-90  60-90  70-90  MeI <10 <5 <3 MeAc 1-20 1-15 1-10

In certain embodiments, as discussed, minor amounts of an alkali component such as KOH can be added to sidedraw 128 via line 160 prior to entering the drying column 130. In other embodiments, the alkali component might also be added to the drying column 130 at the same height level as the stream 128 entering the drying column 130 or at a height above the height level height level as the stream 128 entering the drying column 130. Such addition can neutralize HI in the column.

Drying column bottoms stream 146 preferably comprises or consists essentially of acetic acid. In preferred embodiments, drying column bottoms stream 146 comprises acetic acid in an amount greater than 90 wt. %, e.g., greater than 95 wt. % or greater than 98 wt. %. In embodiments, this stream may also be essentially anhydrous, for example, containing less than 0.2 wt. % water, e.g., less than 0.15 wt. % water, less than 0.12 wt. % water, less than 0.1 wt. % water, or less than 0.05 wt. % water. However, as discussed, the stream may contain varying levels of impurities.

In FIG. 1, the crude acid product is withdrawn as a residue in drying column bottoms stream 146. In embodiments, bottoms stream 146 comprises lithium in an amount of up to or equal to 10 wppm, e.g., up to or equal to 5 wppm, up to or equal to 1 wppm, up to or equal to 500 wppb, up to or equal to 300 wppb, or up to or equal to 100 wppb. In terms of ranges, the crude acid product may comprise lithium in an amount from 0.01 wppm to 10 wppm, e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. In terms of ranges, the side stream 170 may comprise lithium in an amount from 0.01 wppm to 10 wppm, e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. In addition, bottoms stream 146 comprises water in an amount of less than or equal to 0.2 wt. %, e.g., less than or equal to 0.15 wt. %, or less than or equal to 0.12 wt. %, or less than or equal to 0.11 wt. %. In one embodiment, the crude acetic acid product may be essentially anhydrous. In further embodiments, it is preferred not to dilute the crude acid product with an aqueous diluent, such as water.

Thus, in one embodiment there is provided a process for producing acetic acid comprising separating a reaction medium formed in a reactor in a flash vessel to form a liquid recycle and a vapor product stream, distilling the vapor product stream in a first column to obtain a side stream and a low boiling overhead vapor stream comprising water in an amount of greater than or equal to 5 wt. %, condensing the low boiling overhead vapor stream and biphasically separating the condensed stream to form a heavy liquid phase and a light liquid phase, optionally treating a portion of the heavy liquid phase and/or the light liquid phase to remove at least one PRC, distilling the side stream in a second column to obtain a bottoms comprising acetic acid and in an amount of up to or equal to 10 wppm of lithium, contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product, and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid.

As shown in FIG. 2, in certain embodiments, the crude acid product from the drying column 130 may be taken from a side stream 170 at a position slightly above the bottom 172 of the column 130. In one embodiment, side stream 170 is withdrawn within 5 trays from the bottom 172 of the column 130, e.g., within 4 trays from the bottom of the column 130, within 3 trays from the bottom of the column 130, or within 2 trays from the bottom of the column 130. In some embodiments, the side stream 170 is withdrawn a position between 2 and 5 trays from the bottom 172 of the column 130, e.g., a position between 3 and 5 trays from the bottom of the column 130, or position between 3 and 4 trays from the bottom of the column 130. Side stream 170 may be withdrawn in the liquid phase so that the lithium cation would be similar to withdrawing the lithium cation concentration in the drying columns bottoms stream 146. A liquid side stream 170 may be advantageous over a vapor stream in terms of consuming less energy. When a liquid side stream 170 is used then other impurities such as heavy carbonyl containing groups, i.e. propionic acid, may advantageously concentrate in the bottoms stream 174. Residue stream 174 may be discarded or purged from the process 100.

Side stream 170 contains the crude acetic acid product that is sent to cationic exchange resin to remove lithium. In embodiments, side stream 170 comprises lithium in an amount of up to or equal to 10 wppm, e.g., up to or equal to 5 wppm, up to or equal to 1 wppm, up to or equal to 500 wppb, up to or equal to 300 wppb, or up to or equal to 100 wppb. In terms of ranges, the crude acid product may comprise lithium in an amount from 0.01 wppm to 10 wppm, e.g., from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. In terms of ranges, the side stream 170 may comprise lithium in an amount from 0.01 wppm to 10 wppm, e.g., from 0.01 wppm to 8 wppm, from 0.05 wppm to 5 wppm or from 0.05 wppm to 1 wppm. In addition, side stream 170 comprises water in an amount of less than or equal to 0.2 wt. %, e.g., less than or equal to 0.15 wt. %, or less than or equal to 0.12 wt. %, or less than or equal to 0.11 wt. %.

Therefore, in another embodiment, there is provided a process for producing acetic acid comprising separating a reaction medium formed in a reactor in a flash vessel to form a liquid recycle and a vapor product stream, distilling the vapor product stream in a first column to obtain a first side stream and a low boiling overhead vapor stream comprising water in an amount of greater than or equal to 5 wt. %, condensing the low boiling overhead vapor stream and biphasically separating the condensed stream to form a heavy liquid phase and a light liquid phase, optionally treating a portion of the heavy liquid phase and/or the light liquid phase to remove at least one PRC, distilling the first side stream in a second column to obtain a second side stream comprising acetic acid and lithium in an amount of up to or equal to 10 wppm, wherein the second side stream is a liquid, contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product, and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid.

In the present invention, the crude acid product withdrawn from the bottoms or the side draw is further processed to remove lithium derived from and/or generated by the lithium compound in the reaction medium, by passing through cationic exchanger in the acid form and then through metal functionalized iodide removal ion exchange resins, prior to being stored or transported for commercial use. As described herein, cationic exchanger in the acid form are suitable for removing components derived from and/or generated by compounds in the reaction medium that concentrate in the crude acid product. Once these components, and in particular lithium, are removed the iodides may be removed by metal functionalized iodide removal ion exchange resins.

Iodide Removal Beds/Use of Ion Exchange Resins

According to the present process, carboxylic acid streams, e.g., acetic acid streams, that are contaminated with halides (e.g., iodides) and lithium derived from and/or generated by compounds in the reaction medium may be contacted with an acid-form cationic exchange resin followed by a metal-exchanged ion exchange resin having acid cation exchange sites comprising at least one metal selected from the group consisting of silver, mercury, palladium and rhodium under a range of operating conditions. Preferably, the ion exchange resin compositions are provided in fixed beds. The use of fixed iodide removal beds to purify contaminated carboxylic acid streams is well documented in the art (see, for example, U.S. Pat. Nos. 4,615,806; 5,653,853; 5,731,252; and 6,225,498, which are hereby incorporated by reference in their entireties). Generally, a contaminated liquid carboxylic acid stream is contacted with the aforementioned ion exchange resin compositions, by flowing through a series of static fixed beds. The lithium contaminants are removed by the cationic exchanger in the acid form. The halide contaminants, e.g., iodide contaminants, are then removed by reaction with the metal of the metal-exchanged ion exchange resin to form metal iodides. In some embodiments, hydrocarbon moieties, e.g., methyl groups, that may be associated with the iodide may esterify the carboxylic acid. For example, in the case of acetic acid contaminated with methyl iodide, methyl acetate would be produced as a byproduct of the iodide removal. The formation of this esterification product typically does not have a deleterious effect on the treated carboxylic acid stream.

Similar iodide contamination issues may exist in acetic anhydride manufactured via a rhodium-iodide catalyst system. Thus, the inventive process may alternatively be utilized in the purification of crude acetic anhydride product streams.

Suitable acid-form cation exchangers for removing metal ion contaminants in the present invention may comprise strong acid cation exchange resins, for example strong acid macroreticular or macroporous resins, for example Amberlyst® 15 resin (DOW), Purolite C145, or Purolite CT145. The resin may also be an acid-form strong acid cation exchange mesoporous resin. Chelating resins and zeolites may also be used.

Suitably stable ion exchange resins utilized in connection with the present invention for preparing silver or mercury-exchanged resins for iodide removal typically are of the “RSO3H” type classified as “strong acid,” that is, sulfonic acid, cation exchange resins of the macroreticular (macroporous) type. Particularly suitable ion exchange substrates include Amberlyst® 15, Amberlyst® 35 and Amberlyst® 36 resins (DOW) suitable for use at elevated temperatures. Other stable ion exchange substrates such as zeolites may be employed, provided that the material is stable in the organic medium at the conditions of interest, that is, will not chemically decompose or release silver or mercury into the organic medium in unacceptable amounts. Zeolite cationic exchange substrates are disclosed, for example, in U.S. Pat. No. 5,962,735, the disclosure of which is incorporated herein by reference.

At temperatures greater than about 50° C., the silver or mercury exchanged cationic substrate may tend to release small amounts of silver or mercury on the order of 500 wppb or less and thus the silver or mercury exchanged substrate is chemically stable under the conditions of interest. More preferably, silver losses are less than 100 wppb into the organic medium and still more preferably less than 20 wppb into the organic medium. Silver losses may be slightly higher upon start up. In any event, if so desired a bed of acid form cationic material may be placed downstream of the silver or mercury exchange material in addition to the bed of acid form cationic material upstream of the silver or mercury exchange material, to catch any silver or mercury released.

The pressures during the contacting steps with the exchange resins are limited only by the physical strength of the resins. In one embodiment, the contacting is conducted at pressures ranging from 0.1 MPa to 1 MPa, e.g., from 0.1 MPa to 0.8 MPa or from 0.1 MPa to 0.5 MPa. For convenience, however, both pressure and temperature preferably may be established so that the contaminated carboxylic acid stream is processed as a liquid. Thus, for example, when operating at atmospheric pressure, which is generally preferred based on economic considerations, the temperature may range from 17° C. (the freezing point of acetic acid) to 118° C. (the boiling point of acetic acid). It is within the purview of those skilled in the art to determine analogous ranges for product streams comprising other carboxylic acid compounds. The temperature of the contacting step preferably is kept low enough to minimize resin degradation. In one embodiment, the contacting is conducted at a temperature ranging from 25° C. to 120° C., e.g., from 25° C. to 100° C. or from 50° C. to 100° C. Some cationic macroreticular resins typically begin significant degrading (via the mechanism of acid-catalyzed aromatic desulfonation) at temperatures of 150° C. Carboxylic acids having up to 5 carbon atoms, e.g., up to 4 carbon atoms, or up to 3 carbon atoms, remain liquid at these temperatures. Thus, the temperature during the contacting should be maintained below the degradation temperature of the resin utilized. In some embodiments, the operating temperature is kept below temperature limit of the resin, consistent with liquid phase operation and the desired kinetics for lithium and/or halide removal.

The configuration of the resin beds within an acetic acid purification train may vary, but the cationic exchanger should be upstream of the metal-exchanged resin. In a preferred embodiment, the resin beds are configured after a final drying column. Preferably the resin beds are configured in a position wherein the temperature of the crude acid product is low, e.g., less than 120° C. or less than 100° C. The stream contacting the acid-form cationic exchange resin and the stream contacting the metal-exchanged resin can be adjusted to the same or different temperatures. For example, the stream contacting the acid-form cationic exchange resin can be adjusted to a temperature from 25° C. to 120° C., e.g., 30° C. to 100° C., 25° C. to 85° C., 40° C. to 70° C., e.g., 40° C. to 60° C., while the stream contacting the metal-exchanged resin can be adjusted to a temperature from 50° C. to 100° C., e.g., from 50° C. to 85° C., from 55° C. to 75° C., or from 60° C. to 70° C. Aside from the advantages discussed above, lower temperature operation provides for less corrosion as compared to higher temperature operation. Lower temperature operation provides for less formation of corrosion metal contaminants, which, as discussed above, may decrease overall resin life. Also, because lower operating temperatures result in less corrosion, vessels advantageously need not be made from expensive corrosion-resistant metals, and lower grade metals, e.g., standard stainless steel, may be used.

Referring back to FIG. 1, drying column bottoms stream 146 is first passed through cationic exchange resin bed 180 to remove lithium ions. Although one cationic exchange resin bed 180 is shown, it should be understood that a plurality of cationic exchange resin beds may be used in series or parallel. The cationic exchangers may also remove other cations present in the stream, such as potassium, if added via line 160 to drying column 130 as a potassium salt selected from the group consisting of potassium acetate, potassium carbonate, and potassium hydroxide, and corrosion metals. Using the cationic exchangers of the present invention, the intermediate acetic acid product comprises less lithium than the crude acetic acid product. In embodiments, the intermediate acetic acid product contains less than or equal to 5 wt. % of lithium present in crude acetic acid product, e.g., less than or equal to 3 wt. %, less than or equal to 2%, less than or equal to 1.5%, or less than or equal to 1%, less than or equal to 0.5%. Thus, when the crude acetic acid product comprises lithium in an exemplary amount of 400 wppb, the intermediate acid product may comprise less than or equal to 20 wppb lithium. In embodiments, intermediate acid product 182 comprises a reduced amount of lithium in an amount of less than less than or equal to 100 wppb, e.g., less than or equal to 50 wppb, less than or equal to 20 wppb, or less than or equal to 10 wppb. Providing a reduced amount of lithium advantageously maintains the efficient and/or capacity of the metal-exchanged ion exchange resin bed 184 to remove iodides.

The resulting exchanged stream, e.g., intermediate acid product 182, then passes through a metal-exchanged ion exchange resin bed 184 having acid cation exchange sites comprising at least one metal selected from the group consisting of silver, mercury, palladium and rhodium to remove iodides from the stream to produce a purified product 186. Although one metal-exchanged ion exchange resin bed 184 is shown, it should be understood that a plurality of metal-exchanged ion exchange resin beds may be used in series or parallel. In addition to the resin beds, heat exchangers (not shown) may be located before either resin bed to adjust the temperature of the stream 146 and 182 to the appropriate temperature before contacting the resin beds. Similarly in FIG. 2, the crude acetic acid product is fed to cationic exchange resin bed 180 from side stream 170. Heat exchangers or condensers may be located before either resin bed to adjust the temperature of the stream 170 to the appropriate temperature before contacting the resin beds.

In one embodiment, the flow rate through the resin beds ranges from 0.1 bed volumes per hour (“BV/hr”) to 50 BV/hr, e.g., 1 BV/hr to 20 BV/hr or from 6 BV/hr to 10 BV/hr. A bed volume of organic medium is a volume of the medium equal to the volume occupied by the resin bed. A flow rate of 1 BV/hr means that a quantity of organic liquid equal to the volume occupied by the resin bed passes through the resin bed in a one hour time period.

A purified acetic acid composition is obtained as a result of the resin bed treatment. The purified acetic acid composition, in one embodiment, comprises iodides in an amount of less than or equal to 100 wppb, e.g., less than or equal to 90 wppb, less than or equal to 50 wppb, less than or equal to 25 wppb, or less than or equal to 15 wppb. In one embodiment, the purified acetic acid composition comprises lithium in an amount of less than or equal to 100 wppb, e.g., less than or equal to 50 wppb, less than or equal to 20 wppb, or less than or equal to 10 wppb. In terms of ranges, the purified acetic acid composition may comprise from 0 to 100 wppb iodides, e.g., from 0 to 50 wppb, from 1 to 50 wppb, from 2 to 40 wppb; and/or from 0 to 100 wppb lithium, e.g., from 1 to 50 wppb, from 2 to 40 wppb. In other embodiments, the resin beds remove at least 25 wt. % of the iodides from the crude acetic acid product, e.g., at least 50 wt. % or at least 75 wt. %. In one embodiment, the resin beds remove at least 25 wt. % of the lithium from the crude acetic acid product, e.g., at least 50 wt. % or at least 75 wt. %.

One advantage of the present is to reduce a metal displaced from the metal-exchanged ion exchange resin, e.g. silver, mercury, palladium and rhodium, that undesirably accumulate in the purified acetic acid as the final product when no cationic exchanger is used to removed lithium derived from and/or generated by the lithium compound in the reaction medium. In one embodiment, the purified acetic acid comprises a metal displaced from the metal-exchanged ion exchange resin, e.g., silver, mercury, palladium and rhodium, in an amount of less than or equal to 100 wppb, e.g., less than or equal to 90 wppb, less than or equal to 80 wppb, less than or equal to 70 wppb, less than or equal to 60 wppb, less than or equal to 50 wppb, less than or equal to 40 wppb, less than or equal to 30 wppb, or less than or equal to 20 wppb. In terms of ranges, the purified acetic acid comprises a metal displaced from the metal-exchanged ion exchange resin, e.g., silver, mercury, palladium and rhodium, in an amount from 0 to 100 wppb, e.g., from 0.1 to 100 wppb, from 0.5 to 90 wppb, from 1 to 80 wppb, from 1 to 70 wppb, from 1 to 60 wppb, from 1 to 50 wppb, from 1 to 40 wppb, from 1 to 30 wppb, or from 1 to 20 wppb.

In one embodiment, there is provided a process for producing acetic acid comprising carbonylating at least one member selected from the group consisting of methanol, dimethyl ether, and methyl acetate in the presence of water in an amount from 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide and a lithium iodide, to form a reaction medium in a reactor, separating the reaction medium to form a liquid recycle stream and a vapor product stream, separating the vapor product stream in up to 2 distillation columns in a primary purification train to produce a crude acid product comprising acetic acid and lithium derived from and/or generated by the lithium compound in the reaction medium wherein the lithium is in an amount of less than or equal to 100 wppm, contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product comprising lithium in an amount of less than or equal to 100 wppb, provided that the amount of lithium in the intermediate acid product is less than the crude acetic acid product, and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid comprising a metal displaced from the metal-exchanged ion exchange resin in an amount of less than or equal to 100 wppb, lithium in an amount of less than or equal to 100 wppb, and iodide in an amount of less than or equal to less than or equal to 100 wppb.

Distillation

The distillation columns of the present invention may be a conventional distillation column, e.g., a plate column, a packed column, and others. Plate columns may include a perforated plate column, bubble-cap column, Kittel tray column, uniflux tray, or a ripple tray column. For a plate column, the theoretical number of plates is not particularly limited and depending on the species of the component to be separate, may include up to 80 plates, e.g., from 2 to 80, from 5 to 60, from 5 to 50, or more preferably from 7 to 35. The distillation column may include a combination of different distillation apparatuses. For example, a combination of bubble-cap column and perforated plate column may be used as well as a combination of perforated plate column and a packed column.

The distillation temperature and pressure in the distillation system can suitably be selected depending on the condition such as the species of the objective carboxylic acid and the species of the distillation column, or the removal target selected from the lower boiling point impurity and the higher boiling point impurity according to the composition of the feed stream. For example, in a case where the purification of acetic acid is carried out by the distillation column, the inner pressure of the distillation column (usually, the pressure of the column top) may be from 0.01 to 1 MPa, e.g., from 0.02 to 0.7 MPa, and more preferably from 0.05 to 0.5 MPa in terms of gauge pressure. Moreover, the distillation temperature for the distillation column, namely the inner temperature of the column at the temperature of the column top, can be controlled by adjusting the inner pressure of the column, and, for example, may be from 20 to 200° C., e.g., from 50 to 180° C., and more preferably from 100 to 160° C.

The material of each member or unit associated with the distillation system, including the columns, valves, condensers, receivers, pumps, reboilers, and internals, and various lines, each communicating to the distillation system may be made of suitable materials such as glass, metal, ceramic, or combinations thereof, and is not particularly limited to a specific one. According to the present invention, the material of the foregoing distillation system and various lines are a transition metal or a transition-metal-based alloy such as iron alloy, e.g., a stainless steel, nickel or nickel alloy, zirconium or zirconium alloy thereof, titanium or titanium alloy thereof, or aluminum alloy. Suitable iron-based alloys include those containing iron as a main component, e.g., a stainless steel that also comprises chromium, nickel, molybdenum and others. Suitable nickel-based alloys include those alloys containing nickel as a main component and one or more of chromium, iron, cobalt, molybdenum, tungsten, manganese, and others, e.g., HASTELLOY™ and INCONEL™. Corrosion-resistant metals may be particularly suitable as materials for the distillation system and various lines.

As is evident from the figures and text presented above, a variety of embodiments are contemplated.

E1. A process for producing acetic acid comprising:

-   -   carbonylating at least one member selected from the group         consisting of methanol, dimethyl ether, and methyl acetate in         the presence of water in an amount from 0.1 to 14 wt. %, a         rhodium catalyst, methyl iodide, and a lithium compound, to form         a reaction medium in a reactor;     -   separating the reaction medium to form a liquid recycle stream         and a vapor product stream;     -   separating the vapor product stream in up to 2 distillation         columns in a primary purification train to produce a crude acid         product comprising acetic acid and lithium;     -   contacting the crude acetic acid product with a cationic         exchanger in the acid form to produce an intermediate acid         product; and     -   contacting the intermediate acetic acid product with a         metal-exchanged ion exchange resin having acid cation exchange         sites to produce a purified acetic acid.         E2. The process of embodiment E1, wherein the lithium in the         crude acid product is derived from and/or generated by the         lithium compound in the reaction medium.         E3. The process of any one of embodiments E1 or E2, wherein the         crude acid product comprises lithium in an amount of less than         or equal to 10 wppm.         E4. The process of any one of embodiments E1-E3, wherein the         crude acid product comprises water in an amount of less than or         equal to 0.2 wt. %.         E5. The process of any one of embodiments E1-E4, wherein the         intermediate acetic acid product comprises less lithium than the         crude acetic acid product.         E6. The process of any one of embodiments E1-E5, wherein the         intermediate acid product comprises lithium in an amount of less         than or equal to 100 wppb.         E7. The process of any one of embodiments E1-E6, wherein the         purified acetic acid comprises lithium in an amount of less than         or equal to 100 wppb.         E8. The process of any one of embodiments E1-E7, wherein the         purified acetic acid comprises a metal displaced from the         metal-exchanged ion exchange resin in an amount of less than or         equal to 100 wppb.         E9. The process of any one of embodiments E1-E8, wherein the         purified acetic acid comprises iodides in an amount of less than         or equal to 100 wppb.         E10. The process of any one of embodiments E1-E9, wherein the         vapor product stream is separated in 2 distillation columns in         the primary purification train.         E11. The process of any one of embodiments E1-E10, wherein the         cationic exchanger in the acid form comprises a resin of         acid-form strong acid cation exchange macroreticular,         macroporous or mesoporous resins.         E12. The process of any one of embodiments E1-E11, further         comprising a step of adding a potassium salt selected from the         group consisting of potassium acetate, potassium carbonate, and         potassium hydroxide to the distilled acetic acid product prior         to distilling the distilled acetic acid product in a second         distillation column; wherein at least a portion of the potassium         is removed by the cationic exchanger in the acid form.         E13. The process of any one of embodiments E1-E12, further         comprising adjusting the temperature of the crude acetic acid         product to from 50° C. to 120° C.         E14. The process of any one of embodiments E1-E12, further         comprising adjusting the temperature of the intermediate acetic         acid product to from 50° C. to 85° C.         E15. The process of any one of embodiments E1-E14, wherein         separating the vapor product stream comprises distilling the         vapor product stream in a first distillation column to form a         sidedraw comprising acetic acid; and distilling the sidedraw in         a second distillation column to produce a crude acid product         comprising acetic acid and lithium.         E16. The process of embodiment E15, wherein the crude acid         product is removed from a side stream port at a position above         the bottom of the second distillation column.         E17. The process of embodiment E16, wherein the side stream is a         liquid stream.         E18. The process of embodiment E15, wherein the crude acid         product is removed as a residue from the bottom of the second         distillation column.         E19. The process of embodiment E15, further comprising         condensing an low boiling point overhead obtained from the first         distillation column to form a heavy liquid phase and a light         liquid phase, and wherein a portion of the heavy liquid phase is         treated to remove at least one permanganate reducing compound         selected from the group consisting of acetaldehyde, acetone,         methyl ethyl ketone, butylaldehyde, crotonaldehyde, 2-ethyl         crotonaldehyde, 2-ethyl butyraldehyde, and the aldol         condensation products thereof.         E20. A process for producing acetic acid comprising:     -   carbonylating at least one member selected from the group         consisting of methanol, dimethyl ether, and methyl acetate in         the presence of water in an amount from 0.1 to 14 wt. %, a         rhodium catalyst, methyl iodide and iodide salts, to form a         reaction medium in a reactor;     -   separating the reaction medium to form a liquid recycle stream         and a vapor product stream;     -   separating the vapor product stream in up to 2 distillation         columns in a primary purification train to produce a crude acid         product comprising acetic acid comprising at least one cation         selected from the group consisting of Groups IA and IIA of the         periodic table, quaternary nitrogen cations, and         phosphorous-containing cations, wherein the at least one cation         is derived from and/or generated by a compound in the reaction         medium;     -   contacting the crude acetic acid product with a cationic         exchanger in the acid form to produce an intermediate acid         product; and     -   contacting the intermediate acetic acid product with a         metal-exchanged ion exchange resin having acid cation exchange         sites to produce a purified acetic acid.         E21. A process for removing iodides from a liquid composition         comprising:     -   a carboxylic acid or an anhydride thereof, greater than 10 wppb         of C₁₀-C₁₄ alkyl iodides, iodide anions, and a cation selected         from the group consisting of Group IA and IIA metals and         quaternary nitrogen cations, and quaternary         phosphorous-containing cations,     -   contacting said liquid composition with a cationic exchanger in         the acid form to produce an intermediate product with a reduced         concentration of cations selected from the group consisting of         Group IA and IIA metals, quaternary nitrogen cations, and         phosphorous-containing cations; and     -   contacting the intermediate product with a metal-exchanged ion         exchange resin having acid cation exchange sites comprising at         least one metal selected from the group consisting of silver,         mercury, palladium and rhodium to produce a purified acetic acid         product.         E22. The process of any one of embodiments E21-E22, wherein the         cation comprises lithium.         E23. The process of any one of embodiments E1-E22, wherein         metal-exchanged ion exchange resin comprises at least one metal         selected from the group consisting of silver, mercury, palladium         and rhodium.         E24. The process of any one of claims E1-E23, wherein at least         1% of the strong acid exchange sites of said metal-exchanged         resin are occupied by silver.

While the invention has been described in detail, modifications within the spirit and scope of the invention will be readily apparent to those of skill in the art. In view of the foregoing discussion, relevant knowledge in the art and references discussed above in connection with the Background and Detailed Description, the disclosures of which are all incorporated herein by reference. In addition, it should be understood that aspects of the invention and portions of various embodiments and various features recited below and/or in the appended claims may be combined or interchanged either in whole or in part. In the foregoing descriptions of the various embodiments, those embodiments which refer to another embodiment may be appropriately combined with other embodiments as will be appreciated by one of skill in the art. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.

Examples

The present invention will be better understood in view of the following non-limiting examples. A crude acetic acid product was obtained after distilling a vapor product stream in two distillation columns in a primary purification train. The two distillation columns included a light ends column and drying column. The crude acetic acid product was essentially acetic acid and had less than 0.15 wt. % water. The crude acetic acid product was measured for Li and Ag content are in wppb, as shown in the following table. Comparative A-C were run without cationic exchanger in the acid form, which inventive Examples 1-3 were run with a Amberlyst-15 cationic resin. The outlet is the purified product from the metal-ion exchanged resin, namely a Ag-functionalized Amberlyst-15 cationic resin. The high amounts of lithium in the outlet in Comparative A-C correlate to an increase Ag concentration. Once the crude product is passed through the cationic exchanger in the acid form, the lithium concentration decrease significantly, and this results in lower Ag concentrations in the outlet. In Table 4, ND stands for not detectable.

TABLE 4 Outlet from Metal- Crude Exchanged Acetic Cationic Ion Acid Exchanger Exchange Product Outlet Resin Li Ag Li Li Ag Example (wppb) (wppb) (wppb) (wppb) (wppb) A 424 ND — 131 1207 B 346 ND — 267 1415 C 454 ND — 353 1241 1 208 ND ND ND 30 2 422 ND ND ND 25 3 248 ND ND ND 20 

We claim:
 1. A process for producing acetic acid comprising: carbonylating at least one member selected from the group consisting of methanol, dimethyl ether, and methyl acetate in the presence of water in an amount from 0.1 to 14 wt. %, a rhodium catalyst, methyl iodide, and a lithium compound, to form a reaction medium in a reactor; separating the reaction medium to form a liquid recycle stream and a vapor product stream; separating the vapor product stream in up to 2 distillation columns in a primary purification train to produce a crude acid product comprising acetic acid and lithium; contacting the crude acetic acid product with a cationic exchanger in the acid form to produce an intermediate acid product; and contacting the intermediate acetic acid product with a metal-exchanged ion exchange resin having acid cation exchange sites to produce a purified acetic acid.
 2. The process of claim 1, wherein the lithium in the crude acid product is derived from and/or generated by the lithium compound in the reaction medium.
 3. The process of claim 1, wherein the crude acid product comprises lithium in an amount of less than or equal to 10 wppm.
 4. The process of claim 1, wherein the crude acid product comprises water in an amount of less than or equal to 0.2 wt. %.
 5. The process of claim 1, wherein the intermediate acetic acid product comprises less lithium than the crude acetic acid product.
 6. The process of claim 1, wherein the intermediate acid product comprises lithium in an amount of less than or equal to 100 wppb.
 7. The process of claim 1, wherein the purified acetic acid comprises lithium in an amount of less than or equal to 100 wppb.
 8. The process of claim 1, wherein the purified acetic acid comprises a metal displaced from the metal-exchanged ion exchange resin in an amount of less than or equal to 100 wppb.
 9. The process of claim 1, wherein the purified acetic acid comprises iodides in an amount of less than or equal to 100 wppb.
 10. The process of claim 1, wherein the vapor product stream is separated in 2 distillation columns in the primary purification train.
 11. The process of claim 1, wherein the cationic exchanger in the acid form comprises a resin of acid-form strong acid cation exchange macroreticular, macroporous or mesoporous resins.
 12. The process of claim 1, further comprising a step of adding a potassium salt selected from the group consisting of potassium acetate, potassium carbonate, and potassium hydroxide to the distilled acetic acid product prior to distilling the distilled acetic acid product in a second distillation column; wherein at least a portion of the potassium is removed by the cationic exchanger in the acid form.
 13. The process of claim 1, further comprising adjusting the temperature of the crude acetic acid product to from 50° C. to 120° C.
 14. The process of claim 1, further comprising adjusting the temperature of the intermediate acetic acid product to from 50° C. to 85° C.
 15. The process of claim 1, wherein separating the vapor product stream comprises: distilling the vapor product stream in a first distillation column to form a sidedraw comprising acetic acid; and distilling the sidedraw in a second distillation column to produce a crude acid product comprising acetic acid and lithium.
 16. The process of claim 15, wherein the crude acid product is removed from a side stream port at a position above the bottom of the second distillation column.
 17. The process of claim 16, wherein the side stream is a liquid stream.
 18. The process of claim 15, wherein the crude acid product is removed as a residue from the bottom of the second distillation column.
 19. The process of claim 15, further comprising condensing an low boiling point overhead obtained from the first distillation column to form a heavy liquid phase and a light liquid phase, and wherein a portion of the heavy liquid phase is treated to remove at least one permanganate reducing compound selected from the group consisting of acetaldehyde, acetone, methyl ethyl ketone, butylaldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, 2-ethyl butyraldehyde, and the aldol condensation products thereof.
 20. The process of claim 1, wherein metal-exchanged ion exchange resin comprises at least one metal selected from the group consisting of silver, mercury, palladium and rhodium.
 21. The process of claim 1, wherein at least 1% of the strong acid exchange sites of said metal-exchanged resin are occupied by silver. 